The actual vs. planned differences can come from a number of things. Some players have a tendency to shoot more than others and it is not in any of the ratings you see. Also, the opponent matters. If your SG is double-teamed, chances are he won't get as many shots as you'd like. IQs, +/-, and matchups all play into the actual results to different degrees.
One last thing that matters is who else is on the court when a player is out there. One of my bench guys on my Iba team is one of my top scorers this season (he's started 2 or 3 of the 12 games so far). Except for when he started, his distro was about 1/3 of those for the top scorers in the starting line up. But because he plays a lot with the 2nd unit, his relative distribution to the scrubs makes him the clear #1 scoring option at times. He therefore gets a lot of shots and he is able to lead the team in scoring in some games from the bench.